Weekly topic: 5 wealth points for those who answer correctly, 2 wealth points for participation； Question 206

Error: The glass transition temperature Tg is an important characteristic parameter of a material; many of the material’s properties undergo drastic changes around this temperature. Taking glass as an example, at the glass transition temperature, due to changes in the glass structure, many of its physical properties such as heat capacity, density, coefficient of thermal expansion, and electrical conductivity undergo sharp changes within this temperature range. The heat treatment temperature schedule for glass can be accurately determined based on the glass transition temperature. For polymers, it is the temperature at which the polymer transitions from a glassy state to an elastomeric state. At the glass transition temperature, properties such as the specific heat capacity, thermal expansion coefficient, viscosity, refractive index, free volume, and elastic modulus of the polymer undergo sudden changes. From a molecular structure perspective, the glass transition temperature represents a relaxation phenomenon in which the amorphous portion of a polymer transitions from a frozen state to a thawed state; it does not involve phase transition heat like phase transitions do, and therefore it is considered a second-order phase transition (referred to as the primary transition in polymer dynamics). Below the glass transition temperature, polymers are in a glassy state; neither the molecular chains nor the chain segments can move, with only the atoms (or groups) that make up the molecules vibrating at their equilibrium positions ； At the glass transition temperature, although the molecular chains cannot move, the chain segments begin to move, resulting in elastomeric properties; as the temperature rises further, the entire molecular chains start to move, leading to viscous flow properties. 　　The temperature at which a polymer transitions from the elastomeric state to the glassy state is the temperature at which amorphous polymers (including the amorphous portions in crystalline polymers) change from the glassy state to the elastomeric state, or vice versa. It represents the lowest temperature at which the macromolecular chains of amorphous polymers can move freely, and is usually denoted as Tg; this value may vary depending on the methods and conditions used for measurement. An important process parameter for polymers. Above this temperature, the polymer exhibits elasticity ； Below this temperature, polymers exhibit brittleness, which must be taken into account when using them as plastics, rubbers, synthetic fibers, etc. For example, the glass transition temperature of polyvinyl chloride is 80°C. However, he is not the upper limit for the working temperature of the product. For example, the operating temperature of rubber must be above its glass transition temperature; otherwise, it loses its high elasticity.

Judgment: The glass transition temperature is the highest operating temperature for plastics. (V)

The glass transition temperature is the highest operating temperature for plastics. ( √ )

Judgment: The glass transition temperature is the highest operating temperature for plastics. (True)

Error. – Plastics are divided into amorphous plastics (with a maximum usage temperature equal to the glass transition temperature) and crystalline plastics (usually 20–40 degrees below their melting point)

The glass transition temperature is the highest operating temperature for plastics. ( Wrong )

The glass transition temperature is the highest operating temperature for plastics. ( x )

The glass transition temperature is the highest operating temperature for plastics. (Yes)